High power lasers (e.g., 100 kW) are important for use in numerous industrial and military applications. For example, high power lasers are necessary in industrial applications such as materials processing and metal welding. However, increasing the power of the lasers has been impeded by thermal effects which cause the laser beam quality to deteriorate, thereby limiting the effectiveness of the high power laser beam. Prior art solutions to this problem include using solid-state lasers such as fiber lasers. Fiber lasers are less prone to thermal effects on laser beam quality. However, due to the small size of the fiber lasers' cross-sections, fiber laser power scaling has been limited to less than 10 kW due to optical damage and nonlinear effects such as dust, particles in the atmosphere, etc.
A number of techniques may be used to combine the fiber amplifiers (or fiber lasers), such as coherent or incoherent combining techniques. The coherent combining techniques fall into two categories: active phasing and passive phasing. In coherent combining techniques, the beams are coherent and in phase with each other (i.e., locked together). Once such prior attempt to combine a plurality of fiber amplifiers and lasers to increase the power of fiber lasers was made in U.S. Pat. No. 7,274,717. Passive phasing was used along with a fiber combiner to couple a plurality of laser beams' output from individual fiber amplifiers. The drawbacks of this system include a weak feedback signal, which could cause instabilities in the operation of the system. Furthermore, at high power, the system undergoes coupling changes due to thermal effects at the fiber combiner, preventing the scaling to high power.
Active phasing methods include correcting the phase of the individual amplifiers by an electro-optical phase modulator to ensure that the output beams of the individual amplifiers are in phase. Other prior art attempts to combine multiple fiber lasers or amplifiers use active phase detection and control such as in U.S. Pat. No. 6,708,003, and T. M. Shay et al., (Proceedings of the SPEE, Vol. 5550, pp. 313-9 (2004)). A major disadvantage of active phasing techniques is that the system of fiber amplifiers and master oscillator need to be operated at a narrow linewidth ranging from 10 kHz to 100 kHz. This severely limits the power limitation of each fiber amplifier because of Stimulated Brillouin scattering (SBS). This is not an issue in passive phasing because the system runs broadband, which mitigates SBS.
In incoherent combining methods, also called spectral beam combining, the beams do not have to be locked or phased together, but are manipulated by a prism or a grating so that they share a common aperture. Spectral combining has been proposed, for example, in U.S. Pat. Nos. 6,697,192, 6,327,292, 6,208,679, and 6,192,062. A disadvantage of spectral beam combining is that the individual fiber amplifiers are limited in power due to Simulated Brillouin Scattering (SBS). The spectral beam combining method employs gratings, which are very sensitive to thermal effects. Thermal effects cause the beam quality of the output beam to deteriorate. Since the beams have to be aligned and maintained in free-space to angular tolerances of micro-radians, this is an added problem. Accordingly, there is a need for a laser system that can deliver sufficient power levels while retaining high beam quality.